Investigators in the Divisions of Clinical Pharmacology and Cardiology at Duke University Medical Center have used the Rankin Clinical Research Unit to study patients with atrial fibrillation and other common disorders of the heart rhythm. Atrial fibrillation is the most common disorder of the heart rhythm requiring treatment with antiarrhythmic drugs, and it accounts for over one-third of all U.S. hospitalizations for arrhythmias. Patients with atrial fibrillation have their usually normal heart rhythm replaced by periods of rapid, irregular heart beating that may cause a sense of pounding in the chest, shortness of breath, chest pain, dizziness, or loss of consciousness. These periods may last for minutes or hours, or they may last indefinitely unless acute treatment is given by a physician. Azimilide is an important investigational class III antiarrhythmic drug that blocks both rapid and slow inward potassium channels. It is being developed as a therapy to reduce the frequency of symptomatic arrhythmia occurrences in patients with atrial fibrillation, atrial flutter or both. In protocol D88 patients with atrial fibrillation who had successfully completed a randomized, placebo-controlled clinical trial of azimilide were treated with open-label azimilide75 mg or 100 mg daily. Follow-up has been completed, and no further patients are being followed in this protocol.